This invention relates to a process for electrolytically treating the surface of carbon fibers. More particularly, the invention relates to surface-treated carbon fibers and to an electrolytic process for surface-treating carbon fibers. The invention further relates to composites comprising the improved fibers and a bis-maleimide matrix resin having improved mechanical properties.
Strength properties of composite materials and their permanence in adverse environments depend on the strength of the interfacial bonding between the carbon fiber and the resin matrix components. A variety of methods for improving the interfacial bonding are known and are widely available in the fiber art. For example, U.S. Pat. No. 3,671,411 to Ray et al. discloses subjecting a carbon or graphite fiber to an electrolytic reaction in an aqueous electrolyte whereby negative ions are attracted to the surface of the fiber acting as anode, thereby modifying the fiber surface.
U.S. Pat. No. 4,401,533 to Saito et al. discloses electrolytically surface-treating carbon fibers in an aqueous solution of a sulfuric acid salt, passing a current through the fiber at a specified range of current density, while continuously moving the carbon fiber as an anode in the aqueous electrolytic solution.
U.S. Pat. No. 3,832,297 to Paul, Jr. discloses an electrolytic process for surface treating graphite fibers using organic and inorganic ammonium compounds dissolved in water. These compounds are said to decompose substantially completely to gaseous products on heating at temperatures below about 250.degree. C. Illustrative ammonium compounds are stated to include ammonium hydroxide, ammonium carbonate, ammonium bicarbonate, ammonium benzoate, ammonium dithionate, ammonium hydrosulfide, ammonium sulfite, ammonium thiosulfate, and ammonium tartrate.
Although these and other related processes appear to afford some improvement in interfacial bonding, the art continues to have need of composites with still better bonding.